This invention relates to radio frequency power measurement, and more specifically to improvements in directional couplers used to make such measurements.
A directional coupler is a device which measures the power in a wave traveling in a particular direction in a transmission line. Most directional couplers used in conjunction with radio frequency power amplifiers are designed to measure both forward and reflected power. These directional couplers are useful in measuring load conditions, in adjusting the matching between the amplifier output stage and the load, and in protecting the output devices of an amplifier from damage resulting from mismatch.
An early type of directional coupler utilized a secondary transmission line loosely coupled to a primary transmission line at two points spaced from each other by an odd multiple of one quarter wavelength. In this type of directional coupler, the forward wave in the primary line produces a wave which travels in a first direction in the secondary line and which can be measured at a termination at one end of the secondary line. A reflected wave in the primary line produces a wave which travels in the secondary line toward the opposite termination, where it can be measured. This early type of directional coupler is, of course, highly frequency-dependent. Greater bandwidths can be obtained by utilizing more than two coupling points. However, the added coupling increases the complexity of the device.
A second type of directional coupler takes advantage of the fact that the current and voltage in a forward traveling wave in a transmission line are in phase while the current and voltage in the reflected wave are 180.degree. out of phase. The current and the voltage are sampled at both ends of a section of transmission line. In each case, the current sample is converted to a voltage sample which is combined with the voltage sampled at the same end of the line. At the input end of the section of transmission line, the components of the voltage samples which correspond to the voltage and current of the forward wave are added, and the components of the voltage samples which correspond to the voltage and current of the reflected wave are subtractively combined. At the load end of the transmission line section, the components of the voltage samples which correspond to the voltage and current of the forward wave are subtractively combined, and the components of the voltage samples which correspond to the voltage and current of the reflected wave are added. Therefore, the voltage resulting from the addition at the input end is proportional to the forward power, and the voltage resulting from the addition at the load end is proportional to reflected power.
In a typical directional coupler of the second type, the voltage sample is derived through a resistive voltage divider. Unfortunately, physically small resistors in the divider have a limited heat dissipating capability and therefore impose limits on the power handling capacity of the directional coupler. On the other hand, physically larger resistors having a greater heat dissipating capability also have a higher inductance, and impose an upper limit on the frequency range in which the directional coupler can operate. High resistance values would avoid the heat dissipation and inductance problems, but produce erratic voltage samples because of interaction with reactances elsewhere in the circuit.
It is possible to use a pair of capacitors in series as a voltage divider in place of a resistive divider. However, in a typical directional coupler utilizing a capacitive voltage divider, one of the two capacitors, usually the one having the higher capacitance, is shunted by a relatively low resistance branch comprising a milliammeter in series with a resistor. The low resistance branch makes the response of the divider highly frequency-dependent, causing difficulties in calibration and also imposing limits on the frequency range in which the directional coupler can operate.